Fluorescence correlation spectroscopy is a extremely precise tool to measure molecular diffusion coefficients without disturbing the observed system. We measured the diffusion of several different Rhodamine marker dyes in the cytoplasm (CM-TMR) and on the cell membrane (OD-TMR). The diffusion coefficients measured were 5(10-9 cm2/s on the cell membrane and 4(10-8 cm2/s intracellular, however a strong affinity of the label to intracellular substructures could be observed that slowed down diffusion. We investigated several cell lines like RBL, HeLa and EMT6. A Cy-3 labeled IgE receptor could be measured on RBL with 4(10-10 cm2/s.